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Under the Wikipedia entry for Metaldehyde is listed a Melting Point of $\pu{246^\circ C}$ and a Boiling point of $\pu{110^\circ C}$ (actually, it says it sublimes).

How can a Boiling point be lower than the Melting Point? Even if $\pu{110^\circ C}$ is actually a sublimation temperature how can it then subsequently melt at a higher temperature of $\pu{246^\circ C}$?

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Sublimation is not boiling but there is a reason for listing it instead. Sublimation is direct vaporisation from the solid. The most familiar is solid dry ice producing $\ce{CO2}$ gas. So if metaldehyde melts at $\pu{246^\circ C}$ then one might suppose that the boiling point is higher. However, look at the generic $P$ vs. $T$ diagram below. If we assume that at $P = 1000$ is normal atmospheric pressure then a compound with this $P$ vs $T$ sublimes. As we define boiling to occur only at normal pressure and be the liquid to vapour (gas) transition then this compound can never boil at normal pressure. To become a liquid high temperature and pressure is needed. So you can see why the table lists sublimation at normal pressure rather than boiling. P vs T

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    $\begingroup$ yes, exactly. Normal boiling temp is measure at 1 bar . In the illustration if P=1000 this compound will never boil no matter what the temperature is. To get it to boil the pressure would have to be at least approx 5000 and temperature approx 280. $\endgroup$
    – porphyrin
    Commented Jul 24, 2016 at 19:52
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    $\begingroup$ The MP value of 246 C is obtained in a sealed container. $\endgroup$
    – f''
    Commented Jul 24, 2016 at 19:54
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    $\begingroup$ yes, if sealed then the pressure will be large, > 1 bar. There is no getting around the phase diagram. If you can find one for metaldehyde it should be clear, but it will have the same forma as above, just with different values along the axes. $\endgroup$
    – porphyrin
    Commented Jul 24, 2016 at 20:03
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    $\begingroup$ That's another story. Iodine does have a legitimate melting point at normal pressure, though an urban legend says otherwise. $\endgroup$
    – Ivan Neretin
    Commented Jul 24, 2016 at 21:47
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    $\begingroup$ @permeakra Can you post the metaldehyde phase diagram even if blurry? I'd love to see. $\endgroup$
    – curious_cat
    Commented Jul 25, 2016 at 3:52

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